A vapour compression system normally comprises a compressor, a condenser, an expansion device, e.g. in the form of an expansion valve, and an evaporator arranged in a refrigerant path. Refrigerant flowing in the refrigerant path is alternatingly compressed and expanded, and heat exchange takes place in the condenser and in the evaporator. Thereby cooling or heating is provided to a closed volume.
The supply of refrigerant to the evaporator is often controlled on the basis of the superheat of the refrigerant leaving the evaporator. The superheat is the difference between the temperature of the refrigerant leaving the evaporator and the dew point of the refrigerant leaving the evaporator. A high superheat value indicates that the refrigerant leaving the evaporator is gaseous, and that the gaseous refrigerant has been heated in the evaporator. Accordingly, a high superheat value indicates that the potential refrigeration capacity of the evaporator is not utilised in an efficient manner.
On the other hand, zero superheat indicates that liquid refrigerant may be passing through the evaporator and entering the suction line. This is a disadvantage, since it may cause damage to the compressor if liquid refrigerant is allowed to reach the compressor. Therefore it is desirable to operate the vapour compression system in such a manner that the superheat of the refrigerant leaving the evaporator is small, but positive. In order to obtain this, the superheat of the refrigerant leaving the evaporator must be monitored during operation of the vapour compression system. This may, e.g., be done by measuring the temperature of refrigerant entering the evaporator and the temperature of refrigerant leaving the evaporator. As an alternative the temperature and the pressure of refrigerant leaving the evaporator may be measured. In any event, in order to be able to control the vapour compression system to accurately obtain a desired, low superheat value, it is necessary that the sensors used for measuring the temperature and/or pressure of the refrigerant flowing in the refrigerant path are properly calibrated.
JP 2008 202911 discloses a refrigeration apparatus which performs calibration of a refrigerant temperature sensor provided at the inlet and outlet of an evaporator. A control part includes an operation control part for setting the state where the refrigerant is saturated at the inlet and outlet of the evaporator in the operating state of the refrigeration apparatus. A temperature sensor calibration part is arranged to take the detection values of the two refrigerant temperature sensors in the saturation operating state set by the operating control part, and calibrating an error of the refrigerant temperature sensors. A correction control part stores the calibration result of the temperature sensor calibration part, and the detection values of the refrigerant temperature sensors are corrected based on the calibration result. In the method of JP 2008 202911 the evaporator is flooded in order to calibrate the temperature sensors, thereby introducing the risk that liquid refrigerant enters the suction line and reaches the compressor.